docs(openspec): plan shop coupons, accounts, points, flash sales, and group buying

Capture exclusive promotion composition, empty-group cancel, and mall surface scope so the five marketing changes can be implemented without stacking or lifecycle ambiguity.

Co-authored-by: Cursor <cursoragent@cursor.com>

openspec/changes/add-group-buying/specs/frontend-shop-admin/spec.md

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+## ADDED Requirements
+
+### Requirement: Merchant group-buying management
+Shop users SHALL manage only their shop's group-buying activities in shop-admin, configuring an owned SKU, localized display content, minor-unit group price, required paid-member count, active window, and group lifetime. Shop-admin SHALL expose a group-buying navigation entry beside existing shop operations.
+
+#### Scenario: publish activity
+- **WHEN** a merchant publishes a valid group-buying activity
+- **THEN** shoppers can discover it during its active window
+
+#### Scenario: group buying appears in shop navigation
+- **WHEN** an authenticated shop user opens shop-admin
+- **THEN** a group-buying management entry is reachable without leaving the shop-scoped console